1. Field of the Invention
The present invention relates to a high-frequency circuit for use in a low-noise high-frequency amplifier or the like of a low-noise block down-converter used for satellite broadcasting. The present invention, particularly, relates to a high-frequency circuit having bias lines that cross a microstrip line, and to a high-frequency circuit having a plurality of microstrip lines for transmitting a plurality of signals wherein the microstrip lines are arranged to form one or more intersections with one another.
2. Description of the Prior Art
A part of a low-noise high-frequency amplifier used in an LNB (low-noise block down-converter) that receives a signal from a broadcasting satellite or a communications satellite and outputs an intermediate-frequency signal after frequency conversion is shown in FIG. 8 as an example of a conventional high-frequency circuit. A low-noise high-frequency amplifier 10 of the LNB comprises a MIC (microwave integrated circuit) having a substrate, microstrip lines 14L and 14R formed thereon, and elements built in the microstrip lines.
After received by an antenna (not illustrated), radio-frequency signals in a 12 GHz frequency band in the form of left-handed polarized wave and right-handed polarized wave are fed to the microstrip lines 14L and 14R through input terminals 11L and 11R thereof respectively.
The input signal in the form of left-handed polarized wave fed through the input terminal 11L is outputted from an output terminal 12L after having been amplified by two amplifiers 13L1 and 13L2, both of which are built in the microstrip line 14L. The input signal in the form of right-handed polarized wave fed through the input terminal 11R is outputted from an output terminal 12R after having been amplified by two amplifiers 13R1 and 13R2, both of which are built in the microstrip line 14R.
Each of the amplifiers 13L1, 13L2, 13R1, and 13R2 comprises a GaAsFET (Gallium Arsenide Field-Effect Transistor). Between the amplifiers 13L1 and 13L2 in the microstrip line 14L is formed a coupling capacitor C0 for preventing a DC (direct current) component from passing therethrough.
The amplifiers 13L1 and 13L2 are designed to amplify a signal fed to a gate G thereof and output the amplified signal from a drain D thereof when, for example, bias voltages −B1 and +B1 are applied to the gate G and the drain D by way of bias lines 16L1 and 16L2 respectively. In this case, a source of the GaAsFET is connected to ground (not illustrated).
Coupling capacitors C1 to C5 are formed in the microstrip line 14R to separate the bias lines 16L1, 16L2, 16R1, and 16R2 from each other as independent DC lines. The amplifiers 13R1 and 13R2 are designed to amplify a signal fed to a gate G thereof and output the amplified signal from a drain D thereof when, for example, bias voltages −B2 and +B2 are applied to the gate G and the drain D respectively. In this case, a source of the GaAsFET is connected to ground (not illustrated).
However, according to the aforementioned low-noise high-frequency amplifier 10, the microstrip lines 14L and 14R formed on the substrate are made thin and, in addition, a width W thereof is also made smaller so as to increase an overall packaging density. As a result, facing electrodes of each capacitor should be made longer in the longitudinal direction of the microstrip line 14R so that each of the capacitors C1 to C5 has a predetermined capacitance.
Accordingly, the microstrip line 14R becomes longer and, as a result, the bias lines 16L1 and 16L2 are spaced out apart from each other. Consequently, the overall length of the microstrip line 14L also becomes longer, resulting in an unduly larger low-noise high-frequency amplifier 10 in size. A similar drawback is also seen even in a case where the capacitors C1 to C5 can be formed without elongating the facing electrodes thereof in the longitudinal direction of the microstrip line 14R, because the gaps lying between the facing electrodes are added up to an existing length thereof.
Not only such a circuit as the aforementioned low-noise high-frequency amplifier 10, but also any high-frequency circuit having bias lines that cross a microstrip line requires capacitors for separating each bias line as an independent DC line, thereby making the high-frequency circuit still unduly large in size.
To solve the above-mentioned problem, the Japanese Patent Application Laid-Open No. 2002-164701 discloses a structure in which bias lines cross a microstrip line in plan view by routing a part of the bias lines on a reverse side of a substrate.
However, according to the conventional technology disclosed in the Japanese Patent Application Laid-Open No. 2002-164701, a bandwidth of the microstrip line becomes narrower because a ground plane for the microstrip line is made discontinuous. In other words, the conventional technology has a problem of preventing a wider bandwidth necessary for the high-frequency circuit from being realized. Details are described hereunder with reference to FIG. 9.
FIG. 9 is a sectional view showing a portion where a bias line 16L1 and a microstrip line 14R of the conventional low-noise high-frequency amplifier cross each other. It is to be noted that a bias line 16L2 and the microstrip line 14R cross each other in an identical manner. The microstrip line 14R is formed on an obverse side of a substrate 20. Portions of the bias lines 16L1 and 16L2 that are formed on the obverse side are electrically connected continuously to portions of the bias lines 16L1 and 16L2 that are formed on the reverse side by way of through holes 17 respectively. In FIG. 9, a reference numeral 18 is a ground pattern formed on almost an entire reverse side of the substrate 20, and the ground pattern 18 is removed from a portion surrounding the bias lines 16L1 and 16L2 that are formed on the reverse side of the substrate 20.
Accordingly, routing the bias lines 16L1 and 16L2 on a side opposite to a side on which microstrip line 14R is routed means that the ground pattern 18 is disrupted by the bias lines 16L1 and 16L2, causing discontinuity of the ground plane for the microstrip line 14R. This causes a phenomenon in which the bandwidth of the microstrip line 14R is made narrower.
Further, there are cases in which a substrate used for transmitting a plurality of signals has signal lines (microstrip lines) some of which are arranged to cross other signal lines. In one of such cases, crossing means is achieved by connecting portions of one of the microstrip lines with a chip jumper and routing another microstrip line under the chip jumper so that the microstrip lines cross each other. In another of such cases, one of the microstrip lines is routed on the obverse side and another microstrip line is routed on the reverse side so that the microstrip lines cross each other. In either case, the ground plane is discontinued at the crossing region causing the bandwidth to become narrower and a trap to be generated within the used bandwidth. Particularly, crossed microstrip lines often cause adverse effects such as crosstalk of a signal transmitted through one of the microstrip lines into a signal transmitted through another microstrip line. This could cause deterioration in performance.